The present invention relates to [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid crystals having a high bulk density and a process for obtaining the crystals in a high yield. [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid (hereinafter abbreviated as [S,S]-EDDS) is expected to find use as a biodegradative chelating agent for a detergent composition, a photographic bleaching agent, an electroless plating assistant and a peroxide stabilizer, etc.
Several methods are known for a preparation of [S,S]-isomers of ethylenediamine-N,Nxe2x80x2-disuccinic acid, such as (1) a chemical synthetic method in which said isomers are synthesized from L-aspartic acid and dibromoethane in a basic aqueous medium according to Neal and Rose et al. (Inorganic Chemistry, Vol. 7, pp. 2405-2412, 1968), (2) a chemical synthetic method in which the isomers are synthesized from L-aspartic acid and dibromoethane in the presence of calcium hydroxide according to Patel R. N. et al. (WO95-12570), (3) an enzymatic synthetic method in which the isomers are synthesized from fumaric acid and ethylenediamine according to Endo et al., and (4) a fermentative methods using actinomyces according to T. Nishikiori et al. (J. Antibiot. 37, 426-427, 1994) and Z. Hans et al. (WO9636725).
As methods for recovering [S,S]-EDDS, there have been proposed, for example, a method in which an [S,S]-EDDS solution is gradually made acidic with a concentrated hydrochloric acid till reaching the pH of 3.5 and then an objective substance is recovered from the solution according to Neal and Rose et al., a method for recovering from a clacium salt of [S,S]-EDDS (WO96-01801) according to Atkinson. Eldon. E. et al., and a method for recovering from an aqueous solution of [S,S]-EDDS and L-aspartic acid salt according to Atkinson. Eldon. E. et al. (WO96-01802). These proposals relate to techniques for recovering [S,S]-EDDS from an [S,S]-EDDS solution obtained by a chemical synthesis from L-aspartic acid and dibromoethane, and these techniques are intended to minimize an uptake of coprecipitates at the time of a recovery and to obtain a precipitate in such an amount that the precipitate can be easily filtered. Any of these proposals, however, are silent on how to obtain the crystals of [S,S]-EDDS with a high bulk density in a high yield.
According to experiments by the present inventors, the ordinary acid-precipitation method for example which comprises adding a mineral acid at a normal temperature or under cooling to a reaction liquid of [S,S]-EDDS obtained by the enzymatic synthetic method (3) as mentioned above involved problems that precipitated [S,S]-EDDS was extremely high bulky needle crystals, a deterging efficiency was bad, a drying efficiency of the produced crystals was bad, and a transportation efficiency of the crystals was poor because of their large volume per unit weight.
In order to solve the above problems, the present inventors have made extensive studies on crystallization conditions of [S,S]-EDDS such as a temperature, pH and a period of time at the time of the crystallization. In result, the present inventors found out the conditions under which columnar crystals having a high bulk density can be obtained, and quite unexpectedly confirmed that a considerable amount of cyclized products of ethylenediamine-N,Nxe2x80x2-disuccinic acid represented by the following structural formulae (1) and (2) etc. were contained in a mother liquor in which formed crystals were removed, and that certain crystallization conditions promote an intramolecular cyclization reaction to cause a reduction of a recovery yield of [S,S]-EDDS. 
Accordingly, the purpose of the present invention is to suppress a formation of the cyclized products of [S,S]-EDDS at the time of crystallization and simultaneously to solve the various problems encountered in obtaining the [S,S]-EDDS crystals having a high bulk density. Solution of these problems is of vital importance for the industrial production of the crystals of [S,S]-EDDS.
Regarding cyclized products of ethylenediamine-N,Nxe2x80x2-disuccinic acid, those derived from isomeric mixtures thereof ([R,R]-, [S,S]- and [R,S-/S,R]-mixtures) have been reported by Vasil"" ev V.P. et al. (Zh. Neorg. Khim. 34(2), 381-385, 1989) and other researchers. However, there has been report neither on cyclized products derived from optically active [S,S]-isomers alone nor on a formation of the cyclized products at the time of crystallization.
As a result of intensive studies for a solution of the above problems, the present inventors found that it was effective for solving the above problems to conduct a crystallization under specified pH, temperature and period of time conditions, and succeeded in completing the present invention on the basis of this finding.
Thus, the present invention relates to: (1) crystals of [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid having a bulk density of from 0.45 to 1.2 g/cm3; (2) a process for obtaining the crystals described in (1), which comprises the steps of adjusting an aqueous solution of an [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid metal salt to a temperature of from 40 to 80xc2x0 C. and a pH of from 1.9 to 4.5 and then cooling the resulting solution to a temperature below 40xc2x0 C. over a period of from 0.2 to 10 hours while supplying a mineral acid thereto so as to maintain the pH at from 1.9 to 4.5 to precipitate [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid in a high yield; (3) the process for obtaining the crystals described in (1), which comprises the steps of supplying an aqueous solution of an [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid metal salt and a mineral acid into an aqueous solution of an [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid metal salt adjusted to a temperature of from 0 to 40xc2x0 C. and a pH of from 1.9 to 4.5 so as to maintain the pH at from 1.9 to 4.5 and so as to make a residence time from 0.5 to 10 hours, and taking out intermittently or continuously the resulting slurry of [S,S]-ethylenediame-N,Nxe2x80x2-disuccinic acid crystals in a high yield; (4) the process described in (2) or (3), wherein the yield of the crystal is 90% or more; (5) the process described in (4), in which a cyclization of [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid during its crystallization is suppressed; (6) the process described in (2) or (3), wherein the [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid metal salt is a sodium salt and/or a magnesium salt; and (7) the process described in (2) or (3), wherein the mineral acid is a sulfuric acid.
[S,S]-EDDS contemplated in the present invention is the one having a composition that a ratio of [S,S]-isomers to a whole amount of isomers is 90% by weight or more.
The term xe2x80x9cbulk densityxe2x80x9d referred to in the present invention is defined as a value determined according to JIS K5101 for a dry product with a water content (including crystal water) of 15% by weight or less.
The term xe2x80x9cyieldxe2x80x9d referred to in the present invention is a value determined by dividing a weight obtained by subtracting a weight of [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid disappearing at the time of crystallization from a weight of [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid present in an aqueous solution of an ethylenediamine-N,Nxe2x80x2-disuccinic acid salt, by a weight of [S,S]-ethylenediamine-N,Nxe2x80x2-disuccinic acid present in the original aqueous solution, said value being expressed by percent.
Crystallization can be accomplished, for example, in the following way. In the case of a batch process, an aqueous solution of an alkaline metal salt, alkaline earth metal salt or ammonium salt etc. of [S,S]-EDDS with a concentration of from 0.1% by weight to a saturation concentration, obtained by the above-mentioned enzymatic synthetic method from fumaric acid and ethylenediamine (JP-A-9-140390 and EP-A-0805211), is adjusted to a pH in the range of 1.9 to 4.5, preferably 2.5 to 4.0, and a temperature in the range of 40 to 80xc2x0 C., preferably 40 to 60xc2x0 C., and, if necessary after causing a partial precipitation of [S,S]-EDDS or after adding seed crystals of [S,S]-EDDS, is gradually cooled. Cooling temperature is below 40xc2x0 C., preferably 30 to 0xc2x0 C., and cooling time, although variable depending on pH and temperature conditions, is in the range of from 0.2 to 10 hours. In the case of a continuous process, after adjusting the pH to the range of 1.9 to 4.5, preferably 2.4 to 4.0, and the temperature to the range of 0 to 40xc2x0 C., an aqueous solution of an alkaline metal salt, alkaline earth metal salt or ammonium salt etc. of [S,S]-EDDS is supplied thereto so that a residence time become 0.5 to 10 hours, preferably 1 to 5 hours, although variable depending on pH and temperature conditions, and the resulting [S,S]-EDDS crystal slurry is taken out continuously or intermittently.
Although pH rises up gradually with proceeding of a crystallization, pH is adjusted so as to be in a prescribed range by adding an acid as required in this case.
Such pH adjustment can be usually made by adding a mineral acid such as sulfuric acid or hydrochloric acid.
On the other hand, the lower the pH or the higher the temperature or the longer the period of time of an exposure to these conditions, the easier formation of the cyclized products of ethylenediamine-N,Nxe2x80x2-disuccinic acid represented by the structural formulae (1) and (2). The formation of the cyclized product not only causes a reduction of a recovery yield of the [S,S]-EDDS crystals but results in a cause of a reduction of a crystal quality because the cyclized product remaining in a mother liquor adheres to the crystals. However, the use of the above-mentioned preferable conditions can control a less formation of the cyclized product. According to the present invention, it is possible to finally obtain the [S,S]-EDDS crystals in a yield of 90% or more.
Ordinary methods such as filtration, centrifugation, etc., can be employed for collecting the the precipitated crystals. Then, the cyclized product and sulfates etc. formed in the acid-precipitation of crude crystals are washed away with water or an organic solvent. This washing is not subject to any specific restrictions, either, and ordinary washing methods such as rinsing and slurry washing etc. can be employed.
Drying of wet crystals after washing may be conducted at such a temperature that a temperature of the product be 80xc2x0 C. or less.
The present invention is further illustrated by the following examples, but it should be understood that the present invention is not restricted by these examples in any way.